1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having a plurality of regions different in alignment from each other.
2. Description of the Related Art
A liquid crystal display device is widely utilized as a display device such as a wristwatch, an electronic calculator, and the like, since the liquid crystal display device is available as a thin device and can be operated with a low electric voltage. There are known various types of display system, such as a twisted nematic (TN) type liquid crystal display system, and a super-twisted nematic (STN) type liquid crystal display system. With the TN type liquid crystal display system, it is possible to produce a device having an excellent display characteristic which is comparable to that of CRT by incorporating therein an active switching element such as a thin film transistor (TFT). On the other hand, with the STN type liquid crystal display system, it is possible to obtain a device which is capable of high duty multiplex driving. Therefore, the both systems are now much utilized as a display for a word processor or for a personal computer.
To be more specific, a liquid crystal display device of TN type comprises a transparent substrate provided with a driving electrode, a picture element electrode and an alignment layer disposed on the surface thereof for controlling the twist angle and tilt angle of liquid crystal molecules, and a counter substrate provided with an alignment layer, which is positioned to face the above transparent substrate with a narrow space therebetween, thereby forming a liquid crystal cell. A liquid crystal composition is enclosed between these substrates, and a pair of polarizing plates are disposed as the outermost layers, the polarizing direction of each plate being formed to cross each other.
However, the TN type liquid crystal display device is defective in that since alignment directions of the liquid crystal molecules take a twisted structure in the liquid crystal cell, and its manner of rise from the substrate is accompanied with directionality, the color and contrast ratio of the display varies according to the viewing direction, i.e. they are dependent on the visual angle. Moreover, the TN type liquid crystal display device has drawbacks that the angle of visibility of the display is narrow and light utilization efficiency is poor. In particular, since the manner of rise is accompanied with directionality in the case of the TN type liquid crystal display device, there is no fundamental solution to the problem of the narrow angle of visibility.
The effect of directionality in rise of the liquid crystal molecule becomes most prominent in the display of half-tone, so that the direction which causes lowering of contrast and the direction which causes color inversion are directly determined according to the direction of the rise.
To solve this problem, there is known a proposal wherein the direction of rise of liquid crystal molecules on an alignment film is varied within a picture element, thereby mutually compensating to each other any differences in angle of visibility within the planar direction, thereby making the angle of visibility wider. In other words, a method of changing the direction of rise of the liquid crystal molecule by forming a plurality of alignment films different in pre-tilt angle from each other has been proposed therein. However, when different kinds of alignment films are combined, it would give rise to the generation of a direct current component even if the device is driven by an alternating current, thereby causing a drifting phenomenon of the optimum common potential to occur in an active-matrix drive, and also giving rise to the occurrence of a defective display such as flicker and sticking.
There is another proposal to form a plurality of alignment regions, wherein the rise direction of liquid crystal molecule in each region is altered from those in other regions. According to this proposal, an alignment film is first subjected to a rubbing treatment, and then overlaid with a mask pattern through which the exposed portion of the alignment film is further subjected to rubbing treatment in a different direction from that of the previous rubbing treatment. However, this proposal is also confirmed to cause undesirable phenomena, such as the degradation of the alignment film and the lowering of alignment property, due to the influences of various materials such as a solvent contained in a masking material, the masking material itself, a developer used for patterning the mask, and a stripping liquid used for removing the mask. Moreover, there have been found other problems in this method, such as peeling or shaving of the mask due to the poor rubbing resistance found in some kind of masking materials, thus making the selection of the masking material very difficult.